The present invention relates generally to osteosynthetic bone fixation devices, and in particular to a bone screw with a two-part screw head.
Various devices for internal fixation of bone fragments in the human or animal body are known from the related art.
In the case of internal fixation of the spinal column or of parts of the spinal column, such devices often comprise pedicle screws which are anchored by means of threads into the pedicles of the individual vertebrae to be joined, and one or more longitudinal supports which extend in the direction of the spinal column and have to be connected securely to the pedicle screws. To achieve stable anchoring of the whole implant, the pedicle screws must on the one hand be screwed securely into the pedicles and, on the other hand, connected rigidly to the longitudinal supports.
The connection between the screw head of the pedicle screws and the longitudinal support is normally effected by means of clamp mechanisms which permit a stable connection, even at different angles of the pedicle screw in relation to the longitudinal support. The clamp connection may be releasable so that the whole implant can be removed again without leaving large tissue openings in the area of the spinal column. Secure connections between bone screws and plates or supports, which are common in other internal bone fixations, also may be used for internal fixation of the spine so that different angles of the bone screws relative to the plate or the support may be possible without adversely affecting the connections in terms of their stability.
For instance, one bone-anchoring screw and stabilizer rod connection for the internal fixation of vertebrae is known from U.S. Pat. No. 5,466,237 to Byrd (xe2x80x9cByrdxe2x80x9d), which discloses a variable locking stabilizer, anchor seat, and screw. This known invention has a bone-anchoring screw with a screw head, which is designed as a segment of a sphere on its side facing the screw shank and is convex at its end. The spherical segment part of the screw is mounted in a bore of the anchoring element, this bore comprising a concave portion which tapers toward the screw shank so that a ball-joint-type connection is obtained between the bone screw and the anchoring element. This ball-joint-type connection is blocked by tightening a nut on the anchoring element, which nut presses against the longitudinal support which has been placed in the anchoring element and which consequently presses against the terminal convex part of the screw head and thus blocks the screw head in the anchoring element. However, in the case of a screw head which is convex at the end, there is a danger that the means for inserting a screwdriver in the screw head, for example a centrally arranged hexagon socket, will impair the contact surface between a longitudinal support, pressing on the screw head, and the screw head itself.
Despite this development, a need exists for a bone screw with a two-part screw head, where the bottom part of the screw head is securely connected to the screw shank and the top part of the screw head can be connected to the bottom part after implantation of the bone screw so that it is possible for the top surface of a convex screw head to be made smooth in the area of contact with another implant part, for example a longitudinal support, and a punctiform contact can be achieved between the top part of the screw head and the longitudinal support. A need also exists for a stable connection between a bone fastener and an anchoring element which permits different angles between the screw axis and anchoring element and which may permit a form fit with suitable material pairing.
The invention is related to a fastener and a bone fixation assembly for the internal fixation of vertebral bodies. The bone fastener includes a shank having a longitudinal axis, a head positioned adjacent the top end of the shank and having at least two portions separable in the axial direction. The head includes a bottom portion connected to the shank and a top portion connectable to the bottom portion. The bottom portion defines a socket situated coaxially to the longitudinal axis to accept a driver to rotate the fastener, and the top portion has an exterior with a convex design. The top portion at least partially covers the socket when the top portion is connected to the bottom portion.
The bone screw according to the invention comprises a screw shank to be anchored concentric to a longitudinal axis in a bone or bone part, and a likewise concentric, axially two-part screw head. The bottom part of the screw head toward the screw shank has a diameter which is greater than the diameter of the screw shank. The bottom part is designed as a segment of a sphere in the direction toward the screw shank. The top part of the screw head can be fitted on the bottom part or can be connected to the bottom part and screw shank by means of a press fit, a cone connection, a screw connection or a bayonet lock connection.
In the preferred embodiment of the bone screw according to the invention, the bottom part of the screw head and the screw shank are in one piece. In other embodiments, however, the bottom part can also be designed as a separate part, for example as a circular disk which can be pushed over a corresponding peg on the screw shank and which is clamped securely between the screw head and the screw shank when the top part of the screw head is secured.
The diameter of the bottom part of the screw head is preferably between 8 and 10 mm, while the diameter of the screw shank is preferably between 5 and 6 mm.
In a further embodiment of the bone screw according to the invention, the bottom part of the two-part screw head is designed as a circular disk. The thickness of such a circular-disk-shaped bottom part is preferably between 1 and 2 mm. The rim of such a bottom part is preferably stepped and has a lower edge for bearing on the wall of a bore with a curved surface. A linear contact is achieved in this way.
The top part of the screw head can be of convex design at the end, in particular spherical and semi spherical.
Depending on the embodiment, the bone screw according to the invention can be used for the fixation of bones or bone parts in an osteosynthesis fixation device and can serve, for example, for the fixation of bones or bone parts on a bone plate or for the fixation of vertebrae in a spinal column fixation device.
The device according to the invention for osteosynthetic bone fixation comprises at least one bone screw with a screw shank to be anchored in the bone or bone part and a screw head, and at least one fixation body which serves for stable fixation of the bones or bone parts. The fixation body has at least one bore for receiving the bone screw, this bore passing through the fixation body and comprising a concave portion tapering toward the end at the screw shank side. The bottom part of the screw head is in the shape of a segment of a sphere or in the shape of a circular disk, the diameter being dimensioned such that the bottom part, in the concave portion of the bore, can be made to bear on the wall of the bore at different angles between the longitudinal axis of the bone screw and the central axis of the bore. The configuration with a disk-shaped bottom part having a plane surface on the screw shank side, which surface bears on the concave wall of the bore upon tightening of the screw, permits a linear contact between the bone screw and the fixation body.
In one embodiment of the device according to the invention, this is used to connect a longitudinal support to the bone screw, designed as a pedicle screw, within a spinal column fixation system. The fixation body is designed as a receiving head which serves to connect the longitudinal support to the pedicle screw. Besides the through-bore passing through the receiving head in order to receive the pedicle screw, there is additionally a channel extending transverse to the central axis of the receiving head and open toward the screw head side in order to receive the longitudinal support. The device additionally comprises clamping means which can be connected to the receiving head in a releasable manner at the screw head end and serve for fixing the longitudinal support and the pedicle screw within the receiving head. The through-bore comprises a concave portion tapering toward its screw shank end, so that the screw head of the pedicle screw can be made to bear on the wall of the through-bore, in the concave portion of the through-bore, at different angles between the screw axis and the central axis of the bore.
In a further embodiment of the device according to the invention, the bottom part of the screw head has a diameter d, and the concave portion is of spherical design and has a diameter D, where D=d. However, with this design, with a circular-disk-shaped bottom part, only small angles of the screw axis relative to the central axis of the bore in the fixation body are possible, since otherwise the linear contact is obtained only on one part of the collar circumference. For greater angles in the case of circular-disk-shaped bottom parts, a design of the concave portion is suitable with a diameter D, where D greater than d. In this case, the ratio d:D can be chosen between 0.5 and 1.0, preferably between 0.85 and 0.95. In addition, the diameter of the screw head is chosen such that, if the bone screw is in an inclined position, the screw head does not bear on the wall of the bore and thereby restrict an inclined position of the bone screw.
In yet another embodiment of the device according to the invention, the concave portion is designed in the manner of a spherical segment, where the spherical segment has a radius X while the diameter of the concave portion is D, so that X greater than D. The ratio of D/2 to X is between 0.5 and 1.0, preferably between 0.85 and 0.95.
In a particular embodiment of the device according to the invention, the convex top part at the end of the screw head is of spherical or semispherical design. The center of the spherical top part can coincide with the sphere center of the spherical segment on the bottom part. In the case of the connection device between longitudinal support and pedicle screw, the advantage of this design lies in the fact that a longitudinal support clamped between screw head and clamping means presses on the screw head concentric to the central axis even if the pedicle screw is in an inclined position.
The convex screw head is axially in two parts, where the top part at the end of the screw head can be connected releaseably to the bottom part, which is made in one piece with the screw shank. Above all by means of the two-piece design, the means for inserting a screwdriver into the screw shank, for example a hexagon socket or internal thread, can be more easily provided. In addition, in the case of a central arrangement of, for example, a hexagon socket in the bottom part, the bearing between longitudinal support and screw head is not adversely affected by application of the top part after implantation of the bone screw.
In the case of a circular-disk-shaped bottom part, the rim of the bottom part is advantageously stepped, especially on the underside toward the screw shank, so that a lower edge is formed which is intended for linear contact with the wall of the concave portion.
The diameter D of the concave portion and the diameter d of the bottom part are advantageously between 8 and 10 mm. In the case of a circular-disk-shaped bottom part, the latter advantageously has a thickness of 1 to 2 mm.
The external diameter of the screw shank is advantageously 5 to 6 mm. The advantages that may be afforded by the invention include the design of the bone screw, the surface of the convex screw head in the area of contact with another implant part, for example the longitudinal support, is smooth and this contact zone is not impeded by means for receiving a screwdriver.
The advantages that may be afforded by the device according to the invention include the design of the bore for receiving a bone screw and the design of the bone screw with a screw head having a bottom part which is intended to bear in a concave portion of the bore so that, in the case of a circular-disk-shaped bottom part, a linear contact can be obtained which, upon fixation of the device, leads to a secure connection between bone screw and fixation body. In the case of a deformable bore wall, a form-fit connection between the bottom part and bore wall can also be achieved as a result of the linear contact.